Photovoltaic devices generally comprise multiple layers of material deposited on a substrate, such as glass. FIG. 1 depicts a typical photovoltaic device. Photovoltaic device 100 may employ a glass substrate 105, a transparent conductive oxide (TCO) layer 110 deposited on substrate 105, a window layer 115 made from an n-type semiconductor material, an absorber layer 120 made from a semiconductor material, and a metal back contact 125. Typical devices use cadmium telluride (CdTe) as absorber layer 120 and include glass substrate 105, tin oxide (SnO2) or cadmium tin oxide (Cd2SnO4) as TCO layer 110, and cadmium sulfide (CdS) as the window layer 115. By way of example, a deposition process for a typical photovoltaic device on substrate 105 may be ordered as TCO layer 110 including a n-type material doped with one of SnO2 and Cd2SnO4, CdS window layer 115, a CdTe absorber layer 120, and metal back contact 125. CdTe absorber layer 120 may be deposited on top of window layer 115.
An exemplary energy band diagram of a typical thin-film photovoltaic device, such as a CdTe device is depicted in FIG. 2. Band gap energy for F-doped SnO2 as TCO layer is depicted as 205, band gap energy of undoped SnO2 as a buffer layer is depicted as 210, band gap energy of CdS as the window layer is depicted as 215, and band gap energy of CdTe as an absorber layer is depicted as 220. Typically, the conduction band edge offset of CdS relative to CdTe, Δ, is usually −0.2 eV with an experimental uncertainty of +/−0.1 eV.
As depicted in FIG. 2, Δ is the offset in the conduction band edge Ec between the window layer and absorber. In the case of a CdS/CdTe stack, Δ is about −0.2 eV. Theoretical modeling has shown that a more negative Δ leads to larger loss in Voc and FF due to increased rate at which photo carriers recombine at the window/absorber interface. When Δ is made slightly positive (0 to 0.4 eV), the recombination rate can be minimized, leading to improved Voc and FF.
CdS is the conventional window layer in many types of thin-film photovoltaic devices, including photovoltaic devices employing one of CdTe and Cu(In, Ga)Se2 as an absorber layer. However, as depicted in FIG. 2, the optical band gap for CdS is only 2.4 eV.